Many television system scramble systems have been proposed which inject an interfering carrier into the television signal being broadcast. To obtain a picture with good video quality, the user has to filter out the scrambling signal, emulating the response used by the broadcaster. One recent U.S. Pat. No. 4,825,467, issued Apr. 25, 1989, describes an encoded television signal in which one or more interfering characters is injected in specific frequency slots in close proximity to the video carrier. These interfering carriers can be injected either above or below the video carrier, preferably between harmonics of the horizontal frequencies. The interfering carriers are then removed at the receiver by very narrow band rejection filters, constructed of quartz crystal bulk resonators or surface acoustic wave filters. These devices are essentially filters on a chip. A separate filter is required for each interfering carrier injected into the television signal.
A new filter or decoder is required whenever the interfering carrier frequency is changed. This is particularly attractive at the television, because viewers can be required to purchase a removable plug-in type filter to view a selected program on a pay-per-view basis. However, it would be advantageous if there was a universal decoding system for use on mobile receivers that could be used for testing and analyzing the effectiveness of the system at various locations. The broadcaster also might find other effective uses of a universal decoder at the broadcasting end in to improve the system.
It is, therefore, one object of the present invention to provide a universal decoder that can receive and remove one or more interfering carriers injected into a television signal.
Yet another object of the present invention is to provide a system for universally decoding television transmission security systems by converting the spectrum such that the scrambling signal falls in a sharp notch filter. The conversion is done multiple times in cascade to allow for the removal of multiple scrambling signals.
Still another object of the present invention is to provide a decoder in which the television signal is converted such that a scrambling signal falls into a notch of a narrow band rejection filter at a pre-selected frequency after which the signal is again converted back up to its original frequency for viewing.
Yet another object of the present invention is to provide a universal decoder that provides signal detection and a comb filter to remove the scrambling signal or signals.
Yet another object of the present invention is to provide a universal decoder for removing interfering carriers in a television signal which is based on the use of a low-cost video-frequency delay line for removing interfering carriers.
Still another object of the present invention is to provide a universal decoder for removing interfering carriers from a television signal using double comb filters at video frequencies.